Cellulosic and lignocellulosic feedstocks and wastes, such as agricultural residues, wood, forestry wastes, sludge from paper manufacture, and municipal and industrial solid wastes, provide a potentially large renewable feedstock for the production of valuable products such as fuels and other chemicals. Cellulosic and lignocellulosic feedstocks and wastes, composed of carbohydrate polymers comprising cellulose, hemicellulose, and lignin are generally treated by a variety of chemical, mechanical and enzymatic means to release primarily hexose and pentose sugars, which can then be fermented to useful products.
Pretreatment methods are used to make the carbohydrate polymers of cellulosic and lignocellulosic materials more readily available to saccharification enzymes. Standard pretreatment methods have historically utilized primarily strong acids at high temperatures; however due to high energy costs, high equipment costs, high pretreatment catalyst recovery costs and incompatibility with saccharification enzymes, alternative methods are being developed, such as enzymatic pretreatment, or the use of acid or base at milder temperatures where decreased hydrolysis of biomass carbohydrate polymers occurs during pretreatment, requiring improved enzyme systems to saccharify both cellulose and hemicellulose.
U.S. Pat. No. 975,387 relates to a process of treating bagasse with the object to separate therefrom a maximum portion of its papermaking contents with the least expense of materials and time and of a quality suitable for making a commercially useful bagasse paper. The patent discloses that in carrying out the preferred process in which the first main stage thereof comprises two digestive treatments, i.e. fractional digestion, the bagasse is placed in the receptacle and treated for about thirty minutes with a solution containing about from 2.5 to 5 percent of sodium carbonate at about 100° C. or up to and under a steam pressure of ten pounds. The liquid is then run off and the charge is further treated for about 30 minutes with a solution of from 5 percent to 7.5 percent of caustic soda at about the same temperature or steam pressure. Between the two stages the material should be washed for the purpose above explained by running water through it.
U.S. Pat. No. 1,887,241 relates to a digesting or cooking process of treating wood chips for the purpose of producing fiber or pulp, said process to consist of more than one stage. The patent discloses the use of a comparatively cheap chemical, sodium carbonate, in weak solution in the first stage, and in the second stage a solution of caustic soda which is weaker than that now commonly used in the “soda process”. By so doing, it is possible to obtain a larger yield of pulp, from one to five percent, and a greater economy in the use of chemicals, than has been possible by the usual method of using caustic soda only.
A method for producing sugars is needed which selectively removes only lignin without significant loss of either glucan or xylan from the biomass, as these constitute the source of sugars for fermentation. In order to be economically competitive, a commercial process for the production of sugars from renewably-sourced biomass requires the hydrolysis of carbohydrates in lignocellulosic biomass to provide high yields of sugars. An economic process which provides good yields of sugars such as glucose and xylose through both delignification and saccharification is desired. Further desired is a process which provides a saccharification product in good yield and with reduced use of costly pretreatment ingredients, such as sodium hydroxide. Additionally, recycling at least a portion of the chemical waste produced from pretreatment of biomass would be desired.